The development of air bags for use in vehicle applications has been an important safety consideration. Occupant restraint systems have been disclosed in U.S. Pat. Nos. 3,904,222 and 3,887,214. These restraint systems are incorporated into the instrument panel structure. The air bag or inflatable occupant restraint cushion is generally retained in a recessed compartment in the instrument panel.
Closures for instrument panels are disclosed in U.S. Pat. Nos. 4,893,833 and 4,842,299. Each of these patents require many attachments to the instrument panel or have portions of the closure which may fly about the interior compartment of a vehicle during deployment. The use of an appropriately designed closure leads to a need for soft plastic materials that can be readily produced in high volume.
Thermoplastic elastomer blends containing ethylene-propylene thermoplastic block copolymer elastomers as is described in U.S. Pat. No. 4,672,091. Thermoplastic blends of copolymer rubber and polyolefin plastic are described in U.S. Pat. No. 3,835,201. of polyolefin plastics with elastomeric plasticizes are described in U.S. Pat. No. 4,774,277. Polypropylene compositions blended with styrene-ethyl-ene-butylene-styrene block copolymers are described in U.S. Pat. No. 4,871,805.
None of the prior art documents disclose a closure for an occupant restraint system that is comprised of a plurality of molded plastic layers comprised of a mixture of polypropylene and thermoplastic elastomers which layers are of differing Shore hardness and wherein one of the layers has a frangible breaking area which facilitates the opening of the closure by rapid expansion of the inflatable restraint cushion upon deployment.